Extreme ultraviolet lithography (EUVL) systems allow imaging of small-dimensioned features onto substrates and are useful in production of integrated circuits with submicron features. EUVL systems used in the production of integrated circuits have progressed towards shorter optical wavelengths, such as from 248 nm to 193 nm to 157 nm. EUVL systems operating at optical wavelengths at or below 120 nm are currently contemplated. In a typical EUVL system, a condenser system including mirrors collects, shapes, and filters radiation from an extreme ultraviolet (EUV) radiation source to achieve a highly uniform intense beam. The beam is then projected onto a mask containing a pattern to be replicated onto a silicon wafer. The mask reflects the EUV radiation into a reduction imaging system including an assembly of reflective mirrors. The reflective mirrors image the mask pattern and focus the mask pattern onto a resist coating on the silicon wafer. The mask pattern is later transferred to the silicon wafer by etching. For lithographic processes at 248 nm and 193 nm, optical elements such as stepper lenses used in passing light through the mask to form an image of the mask pattern could be made from very pure fused silica. At 157 nm, the fused silica elements are replaced by elements made from Group IIA alkaline earth metal fluorides, such as calcium fluoride, because fused silica has high absorption at 157 nm. For EUVL systems operating at or below 120 nm, there are no known isotropic materials that are transparent at these very short wavelengths. As a result, reflective optics is used instead of conventional focusing optics.
Reflective optics for a EUVL system typically includes a reflective multilayer stack having alternating layers of Molybdenum and Silicon or Molybdenum and Beryllium. The reflective multilayer stack is formed on a substrate having minimum surface roughness. A proposed surface roughness is on the order of less than 0.3 nm rms over 10 mm spacing, and more preferably, less than 0.2 nm rms over 10 μm spacing. An absorber may be formed on the reflective multilayer stack to complete a EUVL mask, where the absorber defines the pattern to be replicated on a wafer, such as a silicon wafer. The substrate for the reflective optics may be made of silicon or glass or other suitable substrate material. In general, it is important that the material used for the substrate has a low coefficient of thermal expansion so that the substrate does not distort under exposure to EUV radiation. It is also important that the material used for the substrate has low absorption at the exposure wavelength. Otherwise, the substrate would heat up and cause distortion and pattern placement errors at the wafer.
The thermal expansion properties of the substrate for EUVL optics and photomasks must be carefully controlled because of the very short wavelengths involved. In particular, it is important that the temperature sensitivity of the coefficient of thermal expansion (CTE) and the range of change of the CTE with temperature are kept as low as possible in the normal operating temperature range of the lithographic process, which is in a general range of 4 to 40° C., preferably 20 to 25° C., with approximately 22° C. being the target temperature. At present, there are only two known commercially available substrate materials that will satisfy the CTE constraints. These are ULE® glass, available from Corning Incorporated, Corning, N.Y., and ZERODUR® glass, available from Schott Lithotec AG, Mainz, Germany. While both are low expansion materials, ULE® glass is a single-phase glass material that is easier to polish. However, despite the advances in low expansion substrate materials, distortion of the written surface of the mask due to heating of the mask during exposure remains a concern. To minimize distortion of the written surface, it is desirable to minimize the thermal gradient through the mask thickness during exposure.